1.A lot of influential people had large investments in slavery.
Many of the founders of the USA and writers of the Constitution opposed slavery -some of these even owned slaves while disapproving of the concept, having inherited slaves but being unable to free them due to the laws related to freeing slaves.)
2.Because neither British nor Americans were much interested in ending it.Both the British and Americans benefited from it.
3.The powerful people never wanted to lose power. They wanted it to become part of their life.Hence slave trade was not to be abolished.
4. Slaves were Africans forcefully kidnapped from Africa.Hence,slave masters believed that Africans were not supposed to be treated as humans.Instead,they wanted to mistreat and punish them while using their energy in forced labour.Therefore, slavery still remained even after the revolution.